fastboot: common: fix default fastboot_boot on 64-bit

When booting on a 64-bit system, the boot_addr_start buffer is not
large enough to contain a 64-bit number, thus leading to a crash
even if fastboot_buf_addr is valid, only the high part of the address
will be printed to boot_addr_start :

fastboot with fastboot_buf_addr = 0x0000000006000000:
  downloading of 92239872 bytes finished
  Booting kernel at 0x00000000...

  "Synchronous Abort" handler, esr 0x96000004
  elr: 00000000010561f4 lr : 0000000001056fac (reloc)
  <snip>
  x28: 000000007df2d38f x29: 000000007df2d1b0

  Resetting CPU ...

With this fix, boot_addr_start can have the full 64-bit address passed
to bootm.

Fixes: f73a7df9 ("net: fastboot: Merge AOSP UDP fastboot")
Cc: Simon Glass <sjg@chromium.org>
Cc: Alex Kiernan <alex.kiernan@gmail.com>
Cc: Sam Protsenko <semen.protsenko@linaro.org>
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Reviewed-by: Simon Glass <sjg@chromium.org>